Conventionally, in a passenger seat for a vehicle, ship or air plane, a reclining device has been provided in order to allow a passenger to suitably tilt a seat back rearward, and a seat belt has been mounted to protect passenger from shock applied to the seat. In general, in the reclining device of this type, seat back frames are rotatably fixed on rear side supporting portions of spreaders forming a base frame, and seat lock devices are linked with the lower end portions of the seat back frames, so that the seat back frames are movable back and forth by extension/contraction of the seat lock devices. In particular, the so-called shoulder harness (hanging type seat belt) is mounted on a passenger seat disposed on the front row for enhancing the safety of a passenger. In the passenger seat mounting tile shoulder harness of this type, a load applied to a passenger upon generation of the shock is supported by the seat back itself through the shoulder harness.
Further, the above seat lock devices constituting the reclining device of the seat can support a large load in tile compression direction, that is, in the direction of turning and tilting the seat back rearwardly; however, can only a small load in the extension direction, that is, in the direction of raising and turning the seat back forward. In particular, in a seat forming brake over arms, the seat back is folded forward in such a state that the seat back frames are fixed by the seat lock devices only by applying of a small load such as 310 bs to the seat back, and thus a large load in the forward direction cannot be supported. In the case that the shoulder harness is mounted on the seat back having such a structure, since the seat back itself cannot support a forward load, the seat back is simply tilted or folded forward by a forward load applied from the shoulder harness to the seat back, which brings about a problem in losing the significance of the mounting of the shoulder harness on the seat back.
Taking the above circumstances into consideration, the present invention has been made, and its object is to provide a shock energy absorbing mechanism in a tilt lock device used for a seat back of a seat, which prevents the forward turning of a seat back by interposing lock levers on shaft supporting portions of seat back frames formed on spreaders for receiving a load applied to the seat back through a shoulder harness; and damps a shock energy transmitted from the seat back to the other member of the seat by the leading edge of the lock levers for absorbing the shock energy without changing the strength or the material of the other portions of the seat.